A study of the production of haemorrhagic injury and paraplegia in rat spinal cord by pulsed ultrasound of low megaHertz frequencies in the context of the safety for clinical usage

Abstract

The spinal cords of adult rats were irradiated with ultrasound using peak intensities of 25 or 50 W cm⁻² at frequencies of 0·5 to 6 MHz. Delivery of energy was pulsed to avoid thermal effects. In most experiments, 10 ms pulses were separated by intervals of 100 ms. Such treatment resulted in paraplegia and/or gross haemorrhage into the cord. The appearance of haemorrhage was found to be a more consistent occurrence and this was used to compare the effects of ultrasound of varying parameters. Damaging ability was maximal at the lowest frequency employed (0·5 MHz)—it decreased with increasing frequency to 5 MHz, at which frequency neither paraplegia nor haemorrhage could be produced. The same method was used to investigate the effects of hypoxia when it was found that an arterial partial pressure of oxygen of 50 mm rendered the tissue more vulnerable to ultrasonic damage by a factor of 40 per cent. The effects of changing the duty cycle were similarly investigated. It was found that haemorrhage occurred whenever an accumulated dose-time had been received which time was characteristic of each frequency and independent of the changed time-averaged intensity resulting from the changed duty cycle. The results are discussed in terms of the known effects of ultrasound, indicating possible action mechanisms. Also considered are the possible relevance of the results to present and future applications of ultrasound in medicine.